Microsoft has 10,000 developers working on the next version of Windows, and even though Vista/7 have been a pretty good improvement on Windows security, you need not look any further than your favorite security site to see that even an army of developers cannot possibly write a perfectly secure OS (not that Microsoft is trying for security as a first priority, though).

The OpenBSD team, on the other hand, has 100-150 or so developers and they DO focus on security and code correctness, and yet they still cannot guarantee third party apps will be secure.